1. Field of the Invention
The present invention relates to an organic light emitting display.
2. Description of the Related Art
Recently, various flat panel displays (FPDs) having reduced weight and volume when compared to cathode ray tubes (CRTs) have been developed. The FPDs include liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), and organic light emitting displays, among others.
Among the FPDs, the organic light emitting displays display images using organic light emitting diodes (OLEDs) that generate light by the re-combination of electrons and holes. The organic light emitting display has high response times and is driven with low power consumption.
FIG. 1 is a circuit diagram illustrating a pixel of a conventional organic light emitting display. In FIG. 1, the transistors included in the pixel are NMOS transistors.
Referring to FIG. 1, a pixel 4 of the conventional organic light emitting display includes an organic light emitting diode (OLED) and a pixel circuit 2 coupled to a data line Dm and a scan line Sn to control the OLED.
The anode electrode of the OLED is coupled to the pixel circuit 2 and the cathode electrode of the OLED is coupled to a second power source ELVSS. The OLED generates light with predetermined brightness corresponding to current supplied from the pixel circuit 2.
The pixel circuit 2 controls the amount of current supplied to the OLED corresponding to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn. Therefore, the pixel circuit 2 includes a second transistor M2 (that is, a driving transistor) coupled between a first power source ELVDD and the OLED, a first transistor M1 coupled between the second transistor M2 and the data line Dm, with a gate electrode coupled to the scan line Sn, and a storage capacitor Cst coupled between a gate electrode and the second electrode of the second transistor M2.
The gate electrode of the first transistor M1 is coupled to the scan line Sn and the first electrode is coupled to the data line Dm. The second electrode of the first transistor M1 is coupled to one terminal of the storage capacitor Cst. Here, the first electrode of the first transistor M1 is set as one of a source electrode or a drain electrode, and the second electrode is the other one of the source electrode or the drain electrode. For example, when the first electrode is the source electrode, the second electrode is the drain electrode. The first transistor M1 is turned on when a scan signal is supplied from the scan line Sn, and supplies a data signal supplied from the data line Dm to the storage capacitor Cst. At this time, the storage capacitor Cst charges a voltage corresponding to the data signal.
The gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst and the first electrode is coupled to the first power source ELVDD. The second electrode of the second transistor M2 is coupled to the other terminal of the storage capacitor Cst and the anode electrode of the OLED. The second transistor M2 controls the amount of current supplied from the first power source ELVDD to the second power source ELVSS via the OLED corresponding to the voltage stored in the storage capacitor Cst.
One terminal of the storage capacitor Cst is coupled to the gate electrode of the second transistor M2 and the other terminal of the storage capacitor Cst is coupled to the anode electrode of the OLED. The storage capacitor Cst charges the voltage corresponding to the data signal.
The conventional pixel 4 supplies a current corresponding to the voltage charged in the storage capacitor Cst to the OLED to display an image with predetermined brightness. However, the above-described conventional organic light emitting display cannot display an image with uniform brightness due to variations in the threshold voltages of the second transistors M2 of the pixels.
That is, when the threshold voltage of the second transistor M2 varies for each of the pixels 4, the pixels 4 generate light with different brightness for a same data signal, and therefore an image with uniform brightness cannot be displayed.